Not applicable.
Not applicable.
1. Field of the Invention
This invention relates to protective coatings for components exposed to high temperatures within a chemically and thermally hostile environment. More particularly, this invention is directed to an anti-stick coating for the internal passages of gas turbine engine components, so as to inhibit the build up within the passages of deposits of adherent contaminants typically entrained in the inlet air of a gas turbine engine.
2. Description of the Related Art
The operating environment within a gas turbine engine is well known to be both thermally and chemically hostile. Nonetheless, higher operating temperatures for gas turbine engines are continuously sought in order to increase their efficiency. However, as operating temperatures increase, the high temperature durability of the components within the hot gas path of the engine must correspondingly increase. Significant advances in high temperature capabilities have been achieved through the formulation of nickel and cobalt-base superalloys. Nonetheless, when used to form components of the turbine, combustor and augmentor sections of a gas turbine engine, superalloys are often susceptible to damage by oxidation and hot corrosion attack and may not retain adequate mechanical properties.
A common solution is to protect the surfaces of such components with an environmental coating, i.e., a coating that is resistant to oxidation and hot corrosion. Coatings that have found wide use for this purpose include diffusion aluminide coatings and overlay coatings such as MCrAlX, where M is nickel, cobalt and/or iron and X is yttrium and/or another rare earth element. During high temperature exposure in air, these coatings form a protective aluminum oxide (alumina) scale that inhibits oxidation of the coating and the underlying substrate. Diffusion aluminide coatings are particularly useful for providing environmental protection to components equipped with internal cooling passages, such as high pressure turbine blades, because aluminides are able to provide environmental protection without significantly reducing the cross-sections of the cooling passages. However, with more advanced cooling designs having complex serpentine passages, certain surfaces may have diminished protection as a result of a very thin aluminide coating.
Hot corrosion of gas turbine engine components generally occurs when sulfur and sodium react during combustion to form sodium sulfate (Na2SO4), which condenses on and subsequently attacks the external component surfaces. Alkali sulfate contaminants also tend to deposit from the inlet air and cause corrosion of the internal cooling surfaces of air-cooled components. Sources of sulfur and sodium for hot corrosion reactions include impurities in the fuel being combusted as well as the intake of alkali laden dust and/or ingestion of sea salt. In the latter situation, hot corrosion typically occurs on hot section turbine blades and vanes under conditions where salt deposits on the component surface as a solid or liquid. The salt deposits can break down the protective alumina scale on a diffusion aluminide coating, resulting in rapid attack of the coating and the underlying superalloy substrate. Hot corrosion produces a loosely adherent external scale with various internal oxides and sulfides penetrating below the external scale. These products are generally sulfur and sodium compounds with elements present in the alloy and possibly other elements from the environment, such as calcium, magnesium, chlorine, etc. As such, hot corrosion products are distinguishable from oxides that normally form or are deposited on gas turbine engine components as a result of the oxidizing environment to which they are exposed.
The internal cooling passages of high pressure turbine components such as blades and vanes have been observed to be particularly susceptible to a build up of contaminants such as silica and alkaline metal-containing compounds, which tend to firmly adhere to the passage walls. The thickness of a layer of these accumulated contaminants can at times be in excess of 0.004 inch (about 0.1 millimeter), which not only reduces cooling flow but is also sufficiently thick to thermally insulate the walls of the cooling passages and reduce the effective height of any turbulence promoters within the passages. As a result, an air-cooled component may experience higher operating temperatures that can significantly reduce its service life. Finally, alkaline metal-containing compounds have been shown to promote hot corrosion attack of cooling passage surfaces during exposures to elevated temperatures.
In view of the above, the surfaces of gas turbine engine components are typically cleaned and, if necessary, their environmental coatings replaced or rejuvenated during engine overhaul and repair. During cleaning, any dirt or other foreign matter that has accumulated in the internal cooling passages of a component is also removed in order to restore cooling flow to an acceptable level and thereby promote the service life of the component. Though coating rejuvenation of gas turbine engine components is becoming more widely practiced, it is very difficult to clean and rejuvenate their internal cooling passages. Removal of contaminants of the type noted above is further complicated if the component has a complex cooling scheme. As a result, a gradual buildup of these contaminants often occurs, with the undesirable effect of reducing cooling efficiency, increasing component operating temperatures, and promoting hot corrosion attack. Vibratory tumbling techniques employed to clean gas turbine engine components have been successful at removing dirt from the external surfaces of gas turbine components, but with little affect on dirt, silica and calcium compounds adhered to the internal cooling passage surfaces of components. Removal of internal dirt and contamination by chemical treatments can be effective but may be impractical for field servicing, and may be aggressive toward the protective diffusion aluminide coating on the cooling passage walls.
From the above, it can be appreciated that the buildup of dirt and contaminants on the internal cooling surfaces of gas turbine engine components is detrimental to the service lives of such components, yet their removal can considerably complicate the cleaning and refurbishing of these components.
The present invention generally provides an anti-stick coating that inhibits the adhesion of contaminants and dirt entrained in the inlet air of a gas turbine engine. More particularly, the anti-stick coating is formed as an outer coating of the internal cooling passages of a gas turbine engine component. The outer coating preferably overlies an environmental coating such as a diffusion aluminide coating formed on the passage surfaces. The outer coating has a thickness of not greater than three micrometers, and is resistant to adhesion by dirt contaminants as a result of comprising at least one layer of tantala (Ta2O5), titania (TiO2), hafnia (HfO2), niobium oxide (Nb2O5), yttria (Y2O3), silica (SiO2) and/or alumina (Al2O3). Because of the internal surfaces being coated, the outer coating is preferably deposited on the environmental coating by a non-line-of-sight deposition process such as chemical vapor deposition (CVD).
Accordance to the invention, the no-stick outer coating significantly inhibits the adhesion of dirt and other contaminants on the walls of internal cooling passages, to the extent that a buildup of such contaminants does not occur or can be more easily removed during cleaning and refurbishment of the component. As a result, the outer coating of this invention is able to maintain an acceptable level of cooling efficiency for the component, thereby minimizing its operating temperatures, as well as inhibits hot corrosion attack during exposures to elevated temperatures. In addition, the outer coating achieves these benefits while eliminating or at least reducing the amount of vibratory tumbling or chemical treatment necessary to remove dirt from the internal surfaces of a component, thereby minimizing potential damage to the protective diffusion aluminide coating by such treatments.
Other objects and advantages of this invention will be better appreciated from the following detailed description.